A Classification of Tasmanian estuaries and assessment of their conservation significance using ecological and physical attributes, population and land use

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Edgar, GJ, Barrett, NS and Graddon, DJ
(1999)
A Classification of Tasmanian estuaries and assessment of their conservation significance using ecological and physical attributes, population and land use.
Technical Report.
Marine Research Laboratories, TAFI, Tasmania.

Abstract

Physical attributes of Tasmanian estuariesA total of 111 estuaries of moderate or large size were recognised around Tasmania and associatedBass Strait islands. The catchments of these estuaries were mapped using GIS, and available data ongeomorphology, geology, hydrology and rainfall collated for each estuary and catchment area.Tasmanian estuaries were classified into nine groups on the basis of physical attributes that includedsalinity and tidal data collected during a field sampling program. The nine groups primarily reflectedthe size of estuaries and their tidal, salinity and rainfall characteristics, and the presence of any seawardbarrier. Catchments in the west, northwest and south of the state were characterised by high rainfall andhigh runoff, while catchments in the east and northeast were relatively dry. Estuaries in northernTasmania possessed much greater tidal ranges than those on the eastern, southern and western coastsand were all open to the sea, whereas many estuaries in eastern Tasmania and the Bass Strait islandswere intermittently closed by sand barriers.Biological attributes of Tasmanian estuariesBaseline information on the abundance, biomass and estimated production of macrobenthicinvertebrate species was collected during a quantitative sampling program at 55 sites in 48 Tasmanianestuaries. These data were generally obtained at three different intertidal levels and two shallowsubtidal depths at each site, and included information on a total of 390 taxa and over 100,000individuals. Data on the distribution of 101 fish species, as obtained during surveys of 75 Tasmanianestuaries using seine nets by Last (1983) with some supplementary sampling, were also incorporatedinto the study.Multivariate analyses indicated that the nine estuarine groups identified using physico-chemicalattributes were useful for categorising faunal relationships between estuaries, although considerablevariation was found between faunas at sites within estuaries, particularly for marine inlet estuaries anddrowned river valleys. Variation in faunal composition between sampling dates at the one site was low.The number of species collected at sites also varied with the estuarine groups identified using physicochemicalattributes, with highest numbers of species occurring in marine inlets and small openestuaries. Relatively few species were collected at sites in microtidal river estuaries, barred low-salinityestuaries or hypersaline lagoons, with extremely low numbers collected in the western TasmanianWanderer estuary. North East Inlet (Flinders Island) and seagrass beds at the mouth of the Tamarestuary possessed exceptionally high diversity for both fishes and invertebrates.The number of macrofaunal species collected at a site was primarily correlated with local salinity andbiomass of submerged plant material, particularly seagrass. Species richness also varied withgeographic location for both macrofauna and fishes, with highest numbers of species occurring in theFurneaux Group, northeastern Tasmania and southeastern Tasmania. Low numbers of species werecollected on the west coast and, to a lesser extent, south coast, central north coast and King Island.These patterns primarily reflected differences in estuary type between regions rather thanconcentrations of locally endemic species.Nearly all fish and invertebrate species recorded from Tasmanian estuaries occurred widely within thestate and have also been recorded in southeastern Australia. Only 1% of estuarine fish species and <5%of invertebrate species were considered endemic to the state. The general pattern of widespread species'ranges around Tasmania was complicated by the absence of most species from the west coast, a small(<10%) component of species that occurred only in the northeast (particularly Flinders Island), and afew localised species that were restricted to different regions of the state.The low number of species recorded from estuaries along the western Tasmanian coast reflectedextremely low macrofaunal productivity in that region. Estimated secondary productivity of west coastestuaries was generally at least one, and up to three, orders of magnitude lower than equivalentestuaries on other coasts. This low productivity was attributed to unusually low concentrations ofdissolved nutrients in rivers and dark tannin-stained waters which greatly restrict algal photosynthesisand primary production.The estimated productivity and biomass of macrofaunal communities were found to vary little betweensites within an estuary compared to variance between estuaries. By contrast, variance in the density ofmacrofauna was much greater between sites within an estuary than between estuaries, and variance wasrelatively low at scales of metres and hundreds of metres at similar tidal heights within a site. Theenvironmental factor most highly correlated with animal density was the biomass of plant material,while estimated faunal productivity and biomass were most highly correlated with salinity.Threats to estuarine biotaBiological resources within most large Tasmanian estuaries are exploited, with unknown consequencesfor ecosystem structure and function. In addition, nine major indirect threats to Tasmanian estuarieshave been recognised: (i) increased siltation resulting from land clearance and urban and rural runoff,(ii) increased nutrient loads resulting from sewage and agricultural use of fertilisers, (iii) urban effluent,(iv) foreshore development and dredging, (v) marine farms, (vi) modification to water flow throughdams and weirs, (vii) acidification of rivers and heavy metal pollution from mines, (viii) the spread ofintroduced pest species, and (ix) long-term climate change. While all of these factors can potentiallydisrupt ecosystem processes, the magnitude and spatial scale of these threats vary greatly.The first seven indirect threats affect individual estuaries and can be ameliorated by changingmanagement practices, whereas substantial global warming would affect all estuarine ecosystemswithin the state through changes to water flow, increased water temperatures and sea level rise. Theeffects of introduced pests are also increasing and uncontrollable at present. Although only fourintroduced species â€“ the green crab Carcinus maenas, the tanaidacean Sinelobus stanfordi, the bivalveTheora lubrica and the gastropod Potamopyrgus antipodarum â€“ were collected during the presentstudy, the threat posed by these and other species (including the seastars Asterias amurensis andPatiriella regularis, the molluscs Musculista senhousia, Crassostrea gigas, Maoricolpus roseus andCorbula gibba, the polychaete Sabella spallanzani and the ricegrass Spartina anglica) was consideredto be extremely high.Amongst the more localised threats to estuaries, siltation, or a correlate thereof, was found to have anextremely widespread effect on Tasmanian estuaries. Estuaries with moderate or high humanpopulation densities in catchments consistently possessed muddy rather than sandy estuarine beds andshores. Although no change in number of macrofaunal species was associated with high humanpopulation densities and associated transformations from sandflats to mudflats, a pronounced shift inthe faunal composition was evident in populated estuaries. These faunal changes were readilydetectable using two disturbance indices described here, DIn and DIp, which are suggested to provideuseful indicators of estuarine health.Assessment of the conservation significance of Tasmanian estuariesHuman population densities within each estuarine catchment and the extent of legislative protectionwere estimated using GIS, census statistics, dwellings marked on 1:25,000 maps and land tenure data.The catchment areas of all Tasmanian mainland estuaries were also categorised in terms of land andvegetation use using GIS and data derived from satellite images. Satellite data for the Bass Straitislands were not available so estuaries in that region were not similarly examined.A total of 24 out of the 90 Tasmanian mainland catchments were considered to be pristine, with littlehuman impact within the catchment. These catchments were nearly all distributed in the south and westof the state and on Cape Barren Island. A small number of catchments were severely impacted byurban development and large scale land clearance, and many others were moderately effected byhuman impacts. The highest levels of land clearance, population and urban development were found incatchments along the south-east, east and north coasts of Tasmania.The conservation significance of each Tasmanian estuary was assessed using the nine groups ofestuaries identified by physical criteria. Within each of the nine groups, estuaries were ranked by levelof anthropogenic disturbance using human population density data, and the estuary with leastdisturbance assigned the highest conservation rank (Class A). Each of these Class A estuaries wastherefore the least disturbed estuary of a particular type, and between them they spanned nearly all ofthe biological and habitat diversity found within estuaries in the state. Where more than one estuarywithin a group was found to be 'pristine', the estuary with highest conservation status was identifiedusing data on the percentage of catchment area included within national parks and crown reserves, anddata on size of estuary.In addition to the nine representative estuaries found to possess highest conservation value, North EastInlet was also assigned Class A conservation status because it possessed high species diversity and included species not contained in other Class A estuaries. The ten class A estuaries are North East Inlet,Black River estuary, Bryans Lagoon, New River Lagoon, Thirsty Lagoon, Tamar River estuary,Southport Lagoon, Bathurst Harbour, Payne Bay and Wanderer River estuary.We recommend that plants, animals and habitats within the ten Class A estuaries and associatedcatchments be protected within an integrated system of Tasmanian estuarine protected areas. We alsorecommend that catchments and aquatic ecosystems of a further 38 estuaries, which were assignedClass B conservation status on the basis of minimal anthropogenic impacts, be quarantined from futuredevelopments, and existing impacts reduced wherever possible.